Fixed Vane vs Variable Geometry Turbochargers...

[Running rough]

What your saying makes a lot of sense, take for example the off road version of the Isx15, (qsx15) the tier4 final emissions engine qsx15 has a monster of a turbo compared to the same year Isx15 say a 2016, of coarse the qsx15 has a lot different torque curve then the on highway Isx15, if you compare the top rated qsx15 with 670 hp and 2050 torque to an x15 605 2050 torque the qsx will eat the x15 for lunch providing you keep the rpms above 1600 rpm, the qsx torque curve drops below 1600 rpm, while the 605 2050 torque x15 would just start to churn more torque as the revs come down, of course a lot of this is tuning but by nature more rpms= more airflow requirements if tuned for higher rpm pulling power, where as a truck engine which normally has a lower load factor then a tractor in the field pulling a large emplement ripping up dirt, so it makes sense a truck engine normally turns slower rpm to increase fuel efficiency, less rpm requires less airflow so therefore a smaller turbo also spools quicker making it a better choice in a truck at normal power levels